ci: restructure workflows to quality/test/luarocks pattern

Problem: CI used a single tests.yml for linting, typechecking, and
testing. No conditional path filtering, no markdown format check, and
a stale mirror_upstream_prs.yml and duplicate luarocks.yml existed.

Solution: replace tests.yml with quality.yaml (stylua, selene,
lua-typecheck, prettier with dorny/paths-filter) and test.yaml
(nvim-busted, stable+nightly matrix). Update luarocks.yaml to
reference quality.yaml. Delete mirror_upstream_prs.yml and duplicate
luarocks.yml. Fix automation workflow sender check.

.github/workflows/automation_remove_question_label_on_comment.yml

@@ -8,7 +8,7 @@ jobs:
   # issues in my "needs triage" filter.
   remove_question:
     runs-on: ubuntu-latest
-    if: github.event.sender.login != 'stevearc'
+    if: github.event.sender.login != 'barrettruth'
     steps:
       - uses: actions/checkout@v4
       - uses: actions-ecosystem/action-remove-labels@v1